1. Field of the Invention
The present invention relates to a method of measuring the thickness of a transparent thin film formed on a substrate and, more particularly, to a technique of measuring the film thickness optically with high accuracy.
2. Description of the Related Arts
Conventionally, various methods have been proposed for optically measuring the thickness of a silicon oxide film formed on a silicon substrate, for example, in an inspection step of semiconductor manufacturing process. The conventional methods are classified into three types according to the principles thereof, which will be referred to as "a polarization analysis method", "a reflected light energy measuring method" and "a reflectance measuring method".
In the polarization analysis method, light is irradiated obliquely to an objective sample, and the thickness of a transparent thin film formed on a substrate in the sample is determined on the basis of change of the polarization state of the reflected light thereof. The change of the polarization state has two meanings, one of which is the change of relative phase shifts between a P-wave component having electric field vector parallel to the plane of incidence and an S-wave component vertical thereto, and the other is the change of amplitude ratio of the both waves.
The polarization analysis method employes a device which is called an ellipsometer, and is an excellent technique capable of measuring a film thickness of even 10 nm or less accurately. However, since it is difficult in this method to measure the film thickness by specifying a minute area as a region to be measured on a sample, the reflected light energy measuring method is adopted in place thereof for such measurement of the film thickness.
In the reflected light energy measuring method, the film thickness is measured on the basis of the characteristic that the energy of the light reflected from the objective sample varies in accordance with the thickness of the transparent thin film. The variation of the light energy is caused by the phenomenon that the light reflected from the surface of the transparent thin film and that reflected from the interface between the transparent thin film and the substrate interfere with each other.
More particularly, this method is conducted as follows. After obtaining correlation data between the reflected light energy and the film thicknesses of the transparent thin films in reference samples having the same optical constants of the transparent thin film and the substrate as the objective sample, the reflected light energy of the objective sample is measured. The correlation data is searched for a film thickness approximately corresponding to the reflected light energy measured. The value of the film thickness thus found is taken as the film thickness of the transparent thin film in the objective sample.
The reflectance measuring method is based on the following principle: The reflectance of the objective sample, expressed as Rs, can be uniquely determined, when following conditions are given: a refractive index n0 of the medium on the incidence side (normally which is air because the film thickness is measured in the atmosphere) of the transparent thin film, an absorptivity k0 thereof, a refractive index n1 of the transparent thin film, an absorptivity k1 thereof, a refractive index n2 of the substrate, an absorptivity k2 thereof, a wavelength .lambda. of the light, an angle of incidence .phi. thereof and the film thickness dx of the transparent thin film. Hence, providing that the values n0, n1, n2, k0, k1, k2, .lambda. and .phi. are specified, the film thickness dx can be determined when the reflectance Rs is defined.
After correlation data between the reflectance Rs and the film thickness dx are obtained for imaginary samples having the same optical constants as the objective sample, the reflectance Rs of the objective sample is measured with a light having the same, wavelength .lambda. and angle of incidence .phi. that are used for obtaining the correlation data. The correlation data is searched for a film thickness dx approxiamtely corresponding to the measured reflectance Rs. The value of the film thickness dx thus found is taken as the film thickness dx of the transparent thin film in the objective sample.
As described above, in the reflected light energy measuring method, since the film thickness is measured on the basis of the correlation data between the reflected light energy and the film thicknesses, it is essential to obtain the correlation data in advance. However, it is not easy to obtain it, as described below.
A measured value of the reflected light energy includes influences of the characteristics peculiar to a device used for the measurement, for example, a photoelectric transfer efficiency of photoelectric converter for detecting light source energy and reflected light energy. Hence, the measured value must obtained experientially as follows:
First of all, a number of reference samples having respectively known different film thicknesses of the transparent thin films and the same optical constants of the transparent thin films and substrates as the objective sample are prepared. The respective reflected light energy of the reference samples is measured with the same device as that of the objective sample. The measured results are used as correlation data between the reflected light energy and the film thicknesses.
Since the correlation data cannot attain sufficient accuracy in measuring the film thickness without collecting data to some extent in detail, a large number of reference samples must be prepared. Accordingly, a large amount of work is necessary for measuring the reflected light energy of the reference samples. Besides, it is not easy but requires a large amount of work to prepare a large number of reference samples.
As aforementioned, the conventional reflected light energy measuring method has a problem in that a large amount of work is necessary for preparation process because the correlation data between the reflected light energy and the film thicknesses must be obtained experientially in advance with the device which is actually used for measuring the objective sample.
Contrary to the energy measuring method, the reflectance measuring method does not have the aforementioned problem, because it is based on the correlation data between the reflectance Rs and the film thickness dx. The value of the reflectance Rs does not include influences of the characteristics peculiar to a device used for measurement. Correlation data between the reflectance Rs and the film thickness dx published in various references are usable. They do not need experiential determination as seen in the case of the correlation data between the reflected light energy Es and the film thickness dx which is employed in the reflected light energy measuring method.
However, it is not easy to measure the reflectance Rs of the objective sample. In measuring the reflectance of the objective sample, various measurement conditions, that is, the refractive index n0 of the medium on the incidence side of the transparent thin film, the absorptivity k0 thereof, the wavelength .lambda. of light and the angle of incidence .phi. thereof must coincide exactly with those used in measuring the reflectance to obtain the correlation data between the reflectance Rs and the film thickness dx in the reference employed.